Conventionally, an electronic control device includes a fuse in case of a fault in the electronic control device. In an electronic control device in which small components are densely arranged, because a short-circuit current generated at a short-circuit fault in the small components does not reach a high current, it takes a long time to interrupt by the fuse. Especially when a large fuse is used for protecting a plurality of electronic control devices so as to reduce the number of fuses and a cost, it takes a longer time. Thus, temperatures of the components may be increased at an interruption and a voltage drop in a power supply wire and the like may be caused for a long time. In contrast, in a common wire, such as a power supply wire (e.g., a battery path and a ground path), that supplies electric power required for operating many circuits and many components mounted in accordance with advancement and diversification of electronic control, a relatively high current flows. Thus, an interrupting current of a large fuse disposed in a common wire path is further increased, and the electronic control device does not secure a sufficient interrupt performance at a short-circuit fault in each circuit or each component. The above-described issue becomes noticeable, for example, in an electronic control device for a vehicle used at a higher temperature and including many mounted devices.
JP-A-2007-311467 discloses a printed circuit board control device in which an interrupt wire is disposed in a power supply wire in each substrate. If an overcurrent flows, the interrupt wire melts and the power supply wire is interrupted in each substrate or each device.
On a substrate in which components are densely mounted, a component-mounted wire, such as a land, on which an electronic component is mounted, and a common wire shared by a plurality of electronic components including the electronic component are disposed adjacent to each other. Thus, usually, a protective layer made of, for example, solder resist is formed on a wire section except a component-coupling portion in the wire. The protective layer is also formed on an interrupt wire disposed between the component-mounted wire and the common wire.
The interrupt wire melts in accordance with heat generated by an overcurrent, and a melt conductor generated by melting of the interrupt wire completely melts down by, such as, expand. Thus, the interrupt wire interrupts the coupling between the component-mounted wire and the common wire. In the above-described interrupt wire, a portion of the melt conductor may not diffuse favorably and may stay. Accordingly, a melt position and a melting time may vary and an interrupt performance of the interrupt wire may be decreased.
In order to form a predetermined pattern for a wire section including an interrupt wire on a substrate in which components are densely mounted, generally, predetermined portions of a conductive layer are covered with resist and the substrate is dipped into an etching liquid. The predetermined portions correspond to the predetermined pattern for the wire section. Thus, the predetermined portions covered with resist are left and other portions are removed by etching.
However, the etching liquid is less likely to flow uniformly at a region around the interrupt wire due to a pattern shape of the wire section. Therefore, the region around the interrupt wire is less likely to be etched, and a wire width of the interrupt wire may vary. In contrast, when the etching liquid is less likely to flow uniformly at the region around the interrupt wire and stays at the region, the region is etched more than necessary and the wire width of the interrupt wire may vary. Thus, a melt position of the interrupt wire and a melting time of the interrupt wire may vary and an interrupt performance of the interrupt wire may be decreased. Specifically, compared with other wires, the interrupt wire is required to have a narrower width at a connecting portion of the interrupt wire and another wire. Thus, a decrease in an interrupt performance of the interrupt wire is more significant at the connecting portion of the interrupt wire and another wire.
Furthermore, when an interrupt wire melts down in accordance with heat generated by an overcurrent, a melt conductor generated by melting of the interrupt wire may break a protective layer that covers the substrate and may flow on the substrate. Thus, electronic components and circuits around the melt conductor may be adversely affected by the melt conductor. Specifically, the melt conductor may cause a short-circuit fault at a densely patterned wire section. Additionally, in a case where the melt conductor adheres to a connecting portion of the substrate and an electronic component, the melt conductor may cause a defect in coupling of the electronic component by melting the solder, which is used to couple the electronic component to the substrate and has a relatively low melting temperature.